Stabilization of a Cationic Amphiphile Monolayer by Polyanions in the

Stabilization of a Cationic Amphiphile Monolayer by Polyanions in the Subphase and Computational Modeling of the Complex at the Air−Water Interface...
1 downloads 0 Views 221KB Size
Download PDF
8118

Langmuir 2001, 17, 8118-8124

Stabilization of a Cationic Amphiphile Monolayer by Polyanions in the Subphase and Computational Modeling of the Complex at the Air-Water Interface Sonika Sharma, M. Sharath Chandra, and T. P. Radhakrishnan* School of Chemistry, University of Hyderabad, Hyderabad - 500 046, India Received April 26, 2001. In Final Form: September 27, 2001 Intermolecular interactions across the air-water interface are the basis for a variety of investigations on monolayers and adsorption phenomena at the interface. We present a systematic study of the influence of different anions in the aqueous subphase on the formation and stabilization of a monolayer of the cationic amphiphile N-octadecyl-4-dimethylaminopyridinium. Some polyanions such as polystyrenesulfonate in the subphase are shown to impart strong stability to the monolayer. The monolayer stabilization through ionic complex formation at the air-water interface is modeled using semiempirical quantum chemical computations. The significance of including solvation effects in the computations is highlighted.

Introduction Intermolecular interactions across the air-water interface have been the focus of a great variety of investigations. H-bonding between amphiphiles spread on the water surface and molecules dissolved in the aqueous subphase has been used to stabilize monolayers,1 promote selective molecular recognition,2 carry out combinatorial surface chemistry,3 and fabricate nanoporous 2-D assemblies.4 Extensive use has been made of strong electrostatic interactions between ionic amphiphiles and ions or polyelectrolytes in the subphase. The classic example is the stabilization of fatty acid monolayers by metal ions in the subphase demonstrated by Blodgett.5 Adsorption of polyelectrolytes from the bulk subphase onto the monolayer assembled at the air-water interface serves as a convenient model system to investigate polyelectrolyte adsorption on solid substrates.6 Specific structural and morphological changes of ionic amphiphile monolayers * Corresponding author. E-mail: [email protected]. Fax: 9140-3012460. (1) (a) Shimomura, M.; Kunitake, T. Thin Solid Films 1985, 132, 243. (b) Erdelen, C.; Laschewsky, A.; Ringsdorf, H.; Schneider, J.; Schuster, A. Thin Solid Films 1989, 180, 153. (c) Chi, L. F.; Johnston, R. R.; Ringsdorf, H. Langmuir 1991, 7, 2323. (d) Goedel, W. A.; Wu, H.; Friedenberg, M. C.; Fuller, G. G.; Foster, M.; Frank, C. W. Langmuir 1994, 10, 4209. (e) Stroeve, P.; van Os, M.; Kunz, R.; Rabolt, J. F. Thin Solid Films 1996, 284-285, 200. (f) Engelking, J.; Menzel, H. Thin Solid Films 1998, 327, 90. (g) Engelking, J.; Ulbrich, D.; Meyer, W. H.; Schenk-Meuser, K.; Duschner, H.; Menzel, H. Mater. Sci. Eng., C 1999, 8-9, 29. (h) Lackmann, H.; Engelking, J.; Menzel, H. Mater. Sci. Eng., C 1999, 8-9, 127. (i) Engelking, J.; Ulbrich, D.; Menzel, H. Macromolecules 2000, 33, 9026. (2) (a) Kawahara, T.; Kurihara, K.; Kunitake, T. Chem. Lett. 1992, 1839. (b) Ahuja, R.; Caruso, P.; Mo¨bius, D.; Paulus, W.; Ringsdorf, H.; Wildburg, G. Angew. Chem., Int. Ed. Engl. 1993, 32, 1033. (c) Bohanon, T. M.; Denzinger, S.; Fink, R.; Paulus, W.; Ringsdorf, H.; Weck, M. Angew. Chem., Int. Ed. Engl. 1995, 34, 58. (d) Talroze, R. V.; Lebedeva, T. L.; Shandryuk, G. A.; Plate, N. A.; Stepina, N. D.; Yanusova, L. G.; Feigin, L. A. Thin Solid Films 1998, 325, 232. (3) Huo, Q.; Sui, G.; Kele, P.; Leblanc, R. M. Angew. Chem., Int. Ed. Engl. 2000, 39, 1854. (4) Plaut, D. J.; Lund, K. M.; Ward, M. D. Chem. Commun. 2000, 769. (5) (a) Blodgett, K. A. J. Am. Chem. Soc. 1934, 56, 495. (b) Blodgett, K. A. J. Am. Chem. Soc. 1935, 57, 1007. (c) Blodgett, K. A. Phys. Rev. 1939, 55, 391. (6) (a) Miyano, K.; Asano, K.; Shimomura, M. Langmuir 2000, 7, 444. (b) Stubenrauch, C.; Albouy, P.; Klitzing, R. v.; Langevin, D. Langmuir 2000, 16, 3206. (c) Engelking, J.; Wittemann, M.; Rehahn, M.; Menzel, H. Langmuir 2000, 16, 3407. (d) Ruths, J.; Essler, F.; Decher, G.; Riegler, H. Langmuir 2000, 16, 8871.

resulting from complexation with polyelectrolytes in the subphase have been shown to facilitate photochemical reactions in the monolayer.7 Biomembrane mimics for ion transport have been fabricated through complexation of cationic amphiphiles with polyanions.8 In most of the investigations of the utilization of intermolecular interactions across the air-water interface, the monolayers already possessed reasonable stability with pure water as the subphase. Hence, the improvement in stability on introduction of the interacting partners in the subphase was usually not very dramatic. On the other hand, it often occurs that a molecule of special interest on derivatization in the traditional way with a long hydrophobic alkyl chain does not form a stable monolayer on the water surface and hence is not amenable to organized assembly through the Langmuir-Blodgett (LB) technique. A common solution to the problem is to mix the amphiphile of interest with other amphiphiles such as fatty acids which are known to form very stable and transferable monolayers.9 However, even though the composite monolayer achieves satisfactory stability, the number density of the useful amphiphile is low, leading to less effective utilization of the monolayer in the application of interest. The introduction of intermolecular interactions across the air-water interface, preferably of the strong Coulombic type discussed above, provides an attractive alternative. The cationic amphiphile N-octadecyl-4-dimethylaminopyridinium (ODP+) has a pushpull chromophore of potential interest in quadratic nonlinear optical applications; adsorption behavior of its soluble analogues having shorter alkyl chains at the air/ water interface has been reported recently.10 ODP+ does not manifest any tendency toward monolayer formation on the water surface when the counterion is the simple bromide ion. Introduction of various types of more complex counterions in the subphase is shown to lead to the formation of monolayers of ODP+ with different levels of (7) (a) Nishiyama, K.; Fujihara, M. Chem. Lett. 1988, 1257. (b) Nishiyama, K.; Kurihara, M.; Fujihara, M. Thin Solid Films 1989, 179, 477. (8) (a) Niwa, M.; Mukai, A.; Higashi, N. Langmuir 1990, 6, 1432. (b) Niwa, M.; Mukai, A.; Higashi, N. Macromolecules 1991, 24, 3314. (9) (a) Shin, D.; Park, M.; Lim, S. Thin Solid Films 1998, 327-329, 607. (b) Munn, R. W.; Szczur, O. Mol. Cryst. Liq. Cryst. 2001, 355, 305 and references therein. (10) Haage, K.; Motschmann, H.; Bae, S.; Gru¨ndemann, E. Colloids Surf., A 2001, 183-185, 583.

10.1021/la010610s CCC: $20.00 © 2001 American Chemical Society Published on Web 11/21/2001

Stabilization of Cationic Monolayers by Polyanions

Figure 1. Molecular structure of ODP+ and polyelectrolytes used in this study.

stability. Specific polyelectrolytes such as polystyrenesulfonate in the subphase are found to lead to very stable monolayers that can also be transferred successfully onto suitable substrates. We show further that semiempirical quantum chemical computations provide a convenient framework to gain insight into the structural aspect of complex formation between the ionic amphiphile and the polyelectrolyte. The significance of including solvation effects in such computations is demonstrated. The counterion system, in addition to being the complexing partner for the amphiphiles, acts as a convenient reference to describe different possible orientations of the amphiphiles at the interface. To the best of our knowledge, this is the first demonstration of the utility of such quantum chemical modeling in the visualization of monolayer-polyelectrolyte complexation at the air-water interface. Experimental and Computational Section ODP+Br- was prepared by the direct reaction of 4-(dimethylamino)pyridine with 1-bromooctadecane in carbon tetrachloride and purified by repeated recrystallizations from acetonitrile. A chloroform solution of ODP+Br- shows a single electronic absorption with λmax at 290 nm. The counterions we have introduced in the subphase range from simple inorganic ions to organic molecular ions and polyelectrolytes. The salts studied are (i) sodium sulfate (Na2SO4), (ii) potassium malonate ((K+)2MA2-), (iii) potassium succinate ((K+)2SC2-), (iv) potassium p-nitrophenolate (K+PNP-), (v) potassium p-toluenesulfonate (K+PTS-), (vi) potassium polyvinyl sulfate ((K+)nPVSn-, MW ) 170 000), (vii) sodium polystyrenesulfonate ((Na+)nPSSn-, MW ) 70 000), (viii) sodium 3:1-poly(styrenesulfonate-co-maleate) ((Na+)n31PSSMn-, MW ∼ 20 000), and (ix) sodium 1:1-poly(styrenesulfonate-co-maleate) ((Na+)n11PSSMn-, MW ∼ 20 000). The polyelectrolytes were purchased from Aldrich Chemical Co. All the salts were purified by multiple recrystallizations from high-purity water and methanol or acetonitrile. The molecular structures of ODP+ and the polyelectrolytes are shown in Figure 1. Pressure-area (π-A) isotherms were investigated on a Nima model 611M LB trough using a Wilhelmy plate for pressure sensing. Typically, ∼1.8 µM solutions of the different salts in high-purity water (Millipore MilliQ) were used as the subphase. A solution of ODP+Br- in chloroform (Uvasol grade, EMerck) was spread on the subphase keeping the mole ratio of ODP+ to the counterions in the subphase ∼1:8; a systematic investigation

Langmuir, Vol. 17, No. 26, 2001 8119 of different ratios in the case of (Na+)nPSSn- showed this to be the optimal one to achieve the best improvement in monolayer stability. All experiments were carried out at 25 °C. After spreading the amphiphile solution, a 30 min wait period was allowed for the adsorption of the polyelectrolyte at the interface and equilibration. The π-A isotherms were recorded using a barrier speed of 50 cm2/min. All pressure-area experiments were repeated on fresh subphases three to five times to confirm reproducibility. Glass and quartz microscope slides with hydrophilic surfaces for the monolayer deposition were prepared as follows; high purity water was used in all operations. The substrate was cleaned with detergent followed by several rinsings in water; it was sonicated in fresh lots of water three times for 30 min each. The surface was made hydrophilic by immersing in aqueous sodium hydroxide for 12 h followed by sonication and rinsing in water. The LB film was coated on the substrate by the vertical dipping procedure employing a dipping speed of 5 mm/ min with the monolayer held at a pressure of 30 mN/m. The specular reflectance (8° incidence) spectrum of the LB film was recorded on a Shimadzu model UV-3100 UV-visible spectrometer using the integrating sphere (ISR 3100) assembly. The sample and the reference uncoated plate were placed with a clean Teflon sheet as the background. The reflectance was converted into absorption using the Kubelka-Munk function. Semiempirical quantum chemical computations on model systems were carried out using the AM1 method11 implemented in the MOPAC93 program package.12 Model systems investigated are (MDP+)n(R5-) complexes where MDP+ represents ODP+ with a methyl group in place of the octadecyl group and R5- represents the pentamer units of PSSn- or PVSn-. Local symmetry imposed on the pentanions is described in the next section. C2v symmetry was imposed locally on the individual MDP+ ions. Using appropriately defined z-matrixes, the MDP+ ions were allowed to rotate and translate freely in the vicinity of the pentanions during the geometry optimizations. The optimizations were carried out with different initial geometries to explore possible pathways on the potential energy surface, and final geometries reached consistently are presented. Geometry optimizations were carried out for the complexes without and with the inclusion of solvation effects modeled using the COSMO subroutine.13 We refer to the former case as the “gas phase” and the latter as the “solvated phase”. Recent studies in our laboratory have shown that the inclusion of solvation effects facilitates the modeling of polymorphism in molecular crystals14 and a realistic description of molecular geometries within crystal environments.15 Since the ODP+-counterion interaction presumably occurs in the water subphase, the solvated phase calculations employed the keyword EPS ) 80 (EPS: dielectric constant) and NSPA ) 40 (NSPA: number of geometric segments per atom). Increasing the value of NSPA to 60 or 70 was found to create negligible changes in the final results. The area/molecule of the MDP+ units was extracted from the optimized geometries as follows. The volume of the cluster was determined using the program MOLDRAW16 and employing the maximum grid of 30 000 points; this program evaluates the molecular volume using van der Waals spheres inscribed around the atoms and taking into account overlap regions. The estimated volumes were reproducible within ∼3 Å3. The height of the clusters was determined as the distance between mean parallel planes of atoms at the top and bottom when the cluster is viewed along the counterion template chain with the polymethylene backbone at the bottom; a correction due to the van der Waals radii of the atoms in the top and bottom planes was included. Area/molecule of ODP+ was computed directly from the volume and height of the clusters. Corrections for the uncomplexed anion groups were included as described at the relevant points in the next section. Figure 2 represents sche(11) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. Soc. 1985, 107, 3902. (12) MOPAC93; Fujitsu Ltd.: Tokyo, Japan, 1993. (13) Klamt, A.; Schu¨u¨rmann, G. J. Chem. Soc., Perkin Trans. 2 1993, 799. (14) Sharma, S.; Radhakrishnan, T. P. J. Phys. Chem. B 2000, 104, 10191. (15) Jayanty, S.; Radhakrishnan, T. P. Chem. Mater. 2001, 13, 2460. (16) Ugliengo, P.; Viterbo, D.; Chiari, G. Z. Kristallogr. 1993, 207, 9. MOLDRAW, release 1.0, version E.; see the website http://www.ch.unito.it/ifm/fisica/moldraw/moldraw.html.

8120

Langmuir, Vol. 17, No. 26, 2001

Sharma et al.

Figure 4. π-A isotherms of ODP+Br- spread on (1) a subphase of pure water and subphases containing (2) (K+)nPVSn-, (3) (Na+)nPSSn-, (4) (Na+)n31PSSMn-, and (5) (Na+)n11PSSMn-; the x-axis refers to area/molecule of ODP+. Figure 2. Schematic drawing of the (MDP+)nPVS5- and (MDP+)nPSS5- clusters considered in the computational study; the top view is shown.

Figure 3. π-A isotherms of ODP+Br- spread on (1) a subphase of pure water and subphases containing (2) K+PNP-, (3) (K+)2SC2-, (4) Na2SO4, (5) (K+)2MA2-, and (6) K+PTS-; the x-axis refers to area/molecule of ODP+. matically the various placements of the MDP+ ion with respect to the counterion template in the clusters investigated.

Results and Discussion ODP+ has a simple push-pull chromophore with a modest hyperpolarizability; the AM1/TDHF17 computed static value for the model headgroup MDP+ is 6 × 10-30 esu. When ODP+Br- is introduced from a chloroform solution on a pure water surface, it does not show any tendency toward uniform spreading. The π-A isotherm 1 in Figure 3 shows that no stable monolayer is formed on compression of the barrier; complete collapse is observed. Isotherms 2-6 in Figure 3 are obtained when ODP+Br- is spread on a subphase containing PNP-, SC2-, SO42-, MA2-, and PTS- ions, respectively. These isotherms suggest that the excess counterions in the subphase facilitate the formation of an ODP+ monolayer; however, (17) Dupuis, M.; Karna, S. J. Comput. Chem. 1991, 12, 487.

the gradual and continuous rise of the pressure down to low values of area/molecule indicates the dominance of a liquidlike regime with poor packing and stability. The subtle differences in the isotherms can be qualitatively correlated to the structure of the specific counterions. Between isotherms 2 and 6, the latter is displaced toward higher areas reflecting the larger size of PTS- relative to that of PNP-. Isotherm 5 in the presence of MA2- appears surprisingly at higher areas compared to isotherm 3 in the presence of SC2-. This suggests that SC2- binds two ODP+ leading to a more efficient packing whereas MA2binds only one. A possible explanation for this observation is that the orientation of the carboxylate groups in the same direction is facilitated when they are attached to an ethylene unit as in SC2- rather than when they are on a methylene unit as in MA2-. Isotherm 4 in the presence of SO42- is quite different from the rest as expected on the basis of its very different structure; we return to this case below. Thus, the isotherms in Figure 3 indicate the formation of ODP+ monolayers with better stabilities than that found on the pure water subphase and relative molecular areas which can be qualitatively correlated to the structures of the relevant counterions in the subphase. Among the different ions, in view of the smaller area/ molecule and steepness of the isotherm, SC2- in the subphase appears to support one of the better packings of ODP+ amphiphiles. Therefore, we have considered the logical extension to the deployment of polyelectrolytes in the subphase. The π-A isotherms recorded for the ODP+ amphiphile with different polyelectrolytes in the subphase are shown in Figure 4 and contrasted with that of ODP+ on pure water. When the polyelectrolyte (K+)nPVSn- is introduced, the isotherm does not improve over those due to the small ions in Figure 3. In fact, isotherm 2 in Figure 4 resembles isotherm 4 in Figure 3, possibly a consequence of the similarity between the binding groups in the two cases. However isotherms 3-5 in Figure 4 due to the polyelectrolytes PSSn-, 31PSSMn-, and 11PSSMn- clearly belong to a new group. These isotherms do not show any liquidlike regime; on the contrary, they represent the onset of a close-packed solid region with a steep rise of pressure around 10-15 mN/m. They also exhibit clear collapse pressures unlike the other isotherms. The area/molecule of ODP+ extracted from the isotherms in the presence of PSSn-, 31PSSMn-, and 11PSSMn- is 25.7, 31.1, and 39.2

Stabilization of Cationic Monolayers by Polyanions

Figure 5. Plots of inverse compressibility versus area/molecule for ODP+ monolayers when the subphase contains (1) Na2SO4, (2) (K+)nPVSn-, and (3) (Na+)nPSSn-; the x-axis refers to area/ molecule of ODP+.

Å2, respectively. Interestingly, the relative displacements are clearly related to the fraction of styrenesulfonate units in the three polymers which are 1.0, 0.75, and 0.5, respectively. This strongly suggests that the ODP+ complexes with the sulfonate unit alone in these polyelectrolytes. This is in good agreement with an earlier observation of selective complexation of a cationic amphiphile with copolymer electrolytes8 and reflects the fact that under the present experimental conditions (pH ∼ 5.5) the carboxylic acid groups are in the protonated state and unavailable for complexation. Though our current focus is on intermolecular interactions across the airwater interface, we note that observations such as this provide very useful insight into the details of polyelectrolyte adsorption onto ionic monolayers. The isotherm in the presence of PVSn- does not show a solid region; however, for the sake of comparison with the other polyelectrolytes, an area/molecule can be extracted by extrapolation of the isotherm near the highest pressures reached. The value obtained is 33.5 Å2. Plots of inverse compressibility (-dπ/dA) shown in Figure 5 reveal the comparison and contrast between the stability of ODP+ monolayers with SO42-, PVSn-, and PSSn- in the subphase. The plots in the first two cases are similar suggesting again that the mode of binding of ODP+ by SO42- resembles that of PVSn-. For area/molecule between 25 and 35 Å2, the inverse compressibility in both cases remains nearly constant at a low value corresponding to the liquidlike regime in the π-A isotherms; the final rise stops around 2 mN/(m Å2). On the contrary, in the case of PSSn- ions in the subphase, the inverse compressibility smoothly rises to a much higher value indicating the formation of the solid region. The higher stability of the monolayers with the PSSn- and its copolymer polyelectrolytes in the subphase is further demonstrated by the area-time plots at constant pressure shown in Figure 6. In the case of the subphase with PVSn-, the monolayer held at a pressure of 18 mN/m shows a slow but steady collapse. However, with PSSn-, 31PSSMn-, and 11PSSMnin the subphase, even at a higher pressure of 30 mN/m the area remains nearly constant for an extended period of time; among the three, there is a minor but clearly discernible improvement in the monolayer stability with increasing content of sulfonate group with the PSSn- case showing negligible collapse. In view of the stability of the ODP+ monolayer on the subphase containing PSSn-, we have attempted transfer of the monolayer onto a suitable

Langmuir, Vol. 17, No. 26, 2001 8121

Figure 6. Constant pressure area-time plots for ODP+ monolayers when the subphase contains (1) (K+)nPVSn- at 18 mN/m and (2) (Na+)nPSSn-, (3) (Na+)n31PSSMn-, and (4) (Na+)n11PSSMn- at 30 mN/m.

Figure 7. Absorption spectra of ODP+Br- (chloroform solution), (Na+)nPSSn- (aqueous solution), and LB film of ODP+-PSSn-.

substrate. Nearly quantitative transfer (transfer ratio ∼ 0.9) of a single layer is achieved when a hydrophilic glass or quartz substrate is used. The electronic absorption spectrum of the LB film on quartz is presented together with the solution spectra of ODP+Br- (in chloroform) and (Na+)nPSSn- (in water) in Figure 7. The presence of ODP+ and PSSn- in the LB film is clearly established by the peaks at 300 and 220 nm, respectively. The two shoulders of the 300 nm band suggest the possible presence of J and H aggregates of the ODP+ amphiphile in the LB film. The small peak near 240 nm appears to be an artifact arising from the Kubelka-Munk transformation of the reflectance spectrum which has a >100% spike at this wavelength. The various experiments prove that PSSn- in the subphase lends high stability to the ODP+ monolayer whereas PVSnperforms poorly. Since both are strong electrolytes with similar anionic groups and high solubility in water, the basis for this difference is not immediately obvious. The higher molecular weight of PVSn- compared to PSSn- used cannot be the cause, since the considerably lower molecular weight polymers 31PSSMn- and 11PSSMn- do not impart any greater stability to the ODP+ monolayer; in fact, the ODP+ monolayer shows slightly poorer stability in the presence of the latter polyelectrolytes compared to PSSn(Figure 6). The hydrophobic nature of PSSn- probably facilitates strong stabilization of the amphiphile mono-

8122

Langmuir, Vol. 17, No. 26, 2001

layer, and the relatively more hydrophilic 31PSSMn-, 11PSSMn-, and PVSn- are less effective. However, among the latter three there is no clear correlation between their stabilizing effect and the hydrophobic nature.18 We have therefore investigated a structural model for the complexation of ODP+ with PSSn- and PVSn- using the computational studies discussed below to gain insight into the factors that influence the stabilization of the amphiphile monolayer. The intrinsic persistence length of polyelectrolytes such as PSSn- is ∼10 Å, and complexation with ionic surfactants is known to cause them to stretch out at the air/solution interface.19 These observations support the following assumptions based on which the clusters represented in Figure 2 were chosen for the computational investigation. (i) The pentameric anion fragment (typically about 10 Å long) serves as a reasonable basic model for the polyelectrolyte keeping in view also the computational effort involved. (ii) The pentanions are likely to adopt a conformation wherein their ionic groups are preferentially oriented upward at the air-water interface so as to facilitate close interaction with the cationic headgroups of the amphiphiles localized at the interface. In fact, when pentanion conformations with the monomer groups oriented parallel or alternately antiparallel were used as the initial geometries, the geometry optimizations led to a low equilibrium wedge angle between them, justifying this assumption. (iii) The ionic headgroup of the amphiphile MDP+ can be positioned in the regions between the anionic groups or above the backbone. In the former case, three cations can be included in vertical (clusters A and B) or horizontal (cluster C) orientations. In the case of PSS5-, cluster C was found to be unviable due to the limited space available between the bulky p-toluenesulfonate groups. When placed over the backbone, the cations are likely to adopt only horizontal orientations (with different possible tilt angles) to facilitate the Coulombic interaction between the strongly positive N+ end of the pyridinium moiety and the anionic groups while accommodating the alkyl chain which is oriented away from the interface; the spatial requirement of such orientations allows the inclusion of only two cations in the clusters D-F. First, we have optimized the pentanions PVS5- and PSS5- alone. A constraint imposed was that the alternate ionic groups on the polymethylene backbone would open symmetrically outward on either side (see Figure 8). This minimizes the repulsions between the groups within the framework of the assumptions listed above. In the case of PSS5-, we have imposed additionally a parallel orientation of the phenyl ring planes. The calculations were carried out in the gas phase and the solvated phase as noted earlier. A significant difference was noted between the two optimized geometries; inclusion of solvation effects leads to a smaller wedge angle between the opened-out monomer groups. In the case of PVS5-, the angles are 26.6° and 14.5° in the gas and solvated phases, respectively; the relevant angles in PSS5- are 68.6° and 54.0°. This clearly demonstrates the amelioration of the electrostatic repulsions between the groups when solvent (18) Compared to PVSn-, the polyanions 11PSSMn- and 31PSSMnpossess additionally phenyl and carboxylic acid groups. The hydrophobicity/hydrophilicity of these groups (extractable from the data in: (a) Eisenberg, D.; Weiss, R. M.; Terwilliger, T. C.; Wilcox, W. Faraday Symp. Chem. Soc. 1982, 17, 109. (b) Dean, P. M. Molecular Foundations of Drug-Receptor Interaction; Cambridge University Press: Cambridge, 1987; p 222.) suggests that PVSn- would be more hydrophobic than the other two. However, PVSn- stabilizes the ODP+ monolayer the least. (19) Langevin, D. Adv. Colloid Interface Sci. 2001, 89-90, 467.

Sharma et al.

Figure 8. AM1 optimized geometries of (a) PVS5- and (b) PSS5in the solvated phase: C (gray), O (red), and S (yellow). Table 1. Relevant Geometric Features of AM1 Optimized Clusters (Figure 2) with PVS5- and PSS5- a wedge angleb (deg) anion cluster PVS5- A B C D E F 5PSS A B D E F

volume (Å3)

G

S

G

S

42.3 41.7 42.3 43.9 43.9 45.2 60.0 60.3 60.0 57.6 61.9

31.4 33.2 43.9 38.8 39.0 42.6 54.7 54.6 56.5 55.3 55.7

883.8 885.3 888.3 732.7 731.8 732.7 1220.4 1227.3 1066.3 1069.3 1070.9

891.8 895.1 886.6 735.6 738.2 734.0 1228.9 1228.4 1072.7 1077.3 1080.1

height area/molecule (Å) (Å2) S S 10.4 12.0 12.6 12.8 11.8 10.2 10.9 11.1 16.1 15.7 14.8

24.6 20.3 19.3 22.6 23.2 29.8 27.5 25.8 26.2 27.0 29.0

a G and S represent the gas phase and solvated phase optimizations (see text for details). b See text for the wedge angles of the uncomplexed pentanions.

effects are taken into account. The optimized geometries for the solvated phase structures are shown in Figure 8. Introduction of MDP+ enhances the wedge angle in PVS5- as shown by the values for the optimized geometries in Table 1. The table also provides the computed volume, effective height, and area/molecule of MDP+ for each of the optimized geometries. In the clusters A-C, a 1:1 ratio between MDP+ and the ionic groups in PVS5- is assumed in the computation of area/molecule, that is, the contribution to the area due to two ionic groups on the latter is subtracted out. In the clusters D-F, the horizontal MDP+ spans nearly two anionic groups; hence the ratio is taken as 1:2 (a case of overcompensation of the amphiphile charge; see ref 20 for another example) so that in the area calculation the contribution of one ionic group in PVS5is subtracted out. For the purpose of these subtractions, the area of one ionic group in PVS5- for each of the clusters was obtained by estimating the cluster volume after the removal of MDP+ ions and estimating the height as described in the earlier section. The results shown in Table (20) Ahrens, H.; Baltes, H.; Schmitt, J.; Mo¨hwald, H.; Helm, C. A. Macromolecules 2001, 34, 4504.

Stabilization of Cationic Monolayers by Polyanions

Langmuir, Vol. 17, No. 26, 2001 8123

Figure 9. AM1 optimized geometries of the clusters (a) A ((MDP+)3PVS5-) and (b) F ((MDP+)2PVS5-) (see Figure 2) in the gas and solvated phases: C (gray), N (blue), O (red), and S (yellow).

1 reiterate the fact that the opening of the wedge angle is relatively less in the solvated phase. Another significant difference between the optimized geometries in the gas and solvated phases is the ordered nature of the MDP+ ions in the latter case in all the clusters studied. This is illustrated by the optimized geometries of some representative clusters in the two phases shown in Figures 9 and 10; it is significant to note that the gas and solvated phase optimizations used the same initial geometry. This strongly suggests that the inclusion of the solvation effect in the computational modeling is essential to obtain a coherent picture of the complexation across the air-water interface. The disorderly structure of the gas phase optimized cluster geometries is incompatible with the ordered nature of monolayers and further makes it impossible to extract any accurate measure of the height of the cluster and hence the area/molecule. Therefore, we have presented in Table 1 the area/molecule calculated for only the solvated phase optimized geometries. Comparison of the computed results in Table 1 with the experimental value for PVSn- in the subphase shows that cluster F (Figure 9) provides the best description of the ODP+-PVSn- complex formation across the air-water interface. Cluster F displays an overcompensation of the amphiphile charge by the polyelectrolyte, and the horizontal orientation of the headgroups of ODP+ dictated by

the polyelectrolyte template most likely leads to a poorly packed monolayer and consequent instabilities described earlier. In contrast to PVS5-, the wedge angles in PSS5- are affected much less by the complexation of MDP+ especially in the solvated phase. This points to the fact that PSSis structurally tuned to complex MDP+. The optimized geometries of the clusters B and D in the gas and solvated phases are shown in Figure 10. The relevant geometric features of the optimized structures are presented in Table 1. Though the area/molecule values computed for both the clusters B and D are close to the experimental value, the former is in better agreement. Further, cluster B with the MDP+ units in the regions between the ionic groups of PSS5-, appropriately oriented for enhanced interaction between the N+ end of pyridinium and the anionic groups, shows smaller distortion of the wedge angle of PSS5-. Finally, the vertical disposition of the ODP+ headgroups in cluster B is likely to lead to a compact packing of the complex and enhanced stability of the ODP+ monolayers on the subphase containing PSSn-. Thus, cluster B appears to be a more appropriate representation of the ODP+PSSn- complex. Our computations point to the significance of the orientation of the headgroup unit as well as the efficiency of complexation with the polyanion in determining the monolayer stability.

8124

Langmuir, Vol. 17, No. 26, 2001

Sharma et al.

Figure 10. AM1 optimized geometries of the clusters (a) B ((MDP+)3PSS5-) and (b) D ((MDP+)2PSS5-) (see Figure 2) in the gas and solvated phases: C (gray), N (blue), O (red), and S (yellow).

Conclusion ODP+Br- which does not exhibit any inclination toward monolayer formation on a pure water surface is found to organize into monolayers of varying stability by the introduction of a variety of counterions in the subphase. Polyelectrolytes such as polystyrenesulfonate in the subphase produce dramatic enhancement of stability of the ODP+ monolayer facilitating quantitative transfer onto suitable substrates. We are currently exploring the nonlinear optical properties of transferred LB films. Semiempirical quantum chemical calculations including solvation effects are shown to provide a useful description of the complexation of the ionic amphiphile and the counterion template across the air-water interface. Subtle distinctions in the stabilization of the ionic amphiphile monolayer brought about by different polyelectrolytes can be understood within the framework of these computational investigations. The present experimental investigation and computational modeling provide a bulk as well as molecular level view of ionic interactions across the

air-water interface and should prove useful in probing the basis of some of the phenomena noted in the Introduction. Acknowledgment. Financial support from the Department of Science and Technology, New Delhi (Swarnajayanti Fellowship), and technical support from Nima Technology, U.K., are gratefully acknowledged. S.S. and M.S. thank the Council of Scientific and Industrial Research, New Delhi, for research fellowships. We thank Professor P. Ugliengo for fruitful discussions regarding the MOLDRAW program. Supporting Information Available: Synthesis and characterization details of ODP+Br-, reflectance and absorption spectra of the LB film, and archive data including optimized geometries from AM1 computations on the solvated phase clusters. This material is available free of charge via the Internet at http://pubs.acs.org. LA010610S